1. Field of the Invention
This invention relates generally to the load bearing prosthetic device suitable for human implant. More particularly, this invention relates to side-dependent prosthetic device fabricated by the use of anisotropically oriented fiber composites to reduce stress shielding and micromotion caused by mismatch of bone/stem properties whereby the stem loosening may be minimized.
2. Description of the Prior Art
Even that total hip replacement using artificial components is an established procedure in orthopedic surgery, there are still major concerns for micromotions and stress shielding resulting from mismatch of bone and stem properties causing stem loosening in clinical studies. More details of the studies are published by Huisides, R, et al. (please refer to Clinical Othop., 274:124-134, 1992). Materials such as stainless steel, cobalt chrome alloy and titanium alloy are commonly used for the design and fabrication of artificial hip prostheses either with or without self curing cement. Recently, uncemented press-fit fixation with low modulus hip stem is being used increasingly as an alternative method to cemented joint replacement due to difficulties experienced with the cemented prostheses. Without the use of cement, bonding at the prosthesis-bone interface is becoming important for the initial fixation as well as long term stability. For that reason, use of different materials, including the advanced fiber-2 reinforcement composite materials for prosthesis application have been actively investigated explored. Because the research in this area is relatively new, there are only few published studies in this field. However, no studies have been performed yet to evaluate the effect of fiber orientation of a composite hip implant on the response of the femoral bone. The clinic studies suggested that mismatch of bone and stem properties often are caused by the side-dependent motion and structural characteristics of the stem when the prosthetic devices are applied for a hip implant. However, none of the prior art techniques provide a solution to these concerns. Specifically, several US patents are issued applying fiber composite materials for fabrication of orthopedic devices. However, as explained below, these inventions employ typical balanced composite materials. These types of prostheses would be adequate for applications to replace parts of the body, e.g., neck or spine segments, where no side-dependent considerations are required.
Please refer to FIGS. 1A for a perspective view of a prosthesis that includes a shaft and a neck. FIG. 1B is an explosive view of a segment of the laminated composite hip prosthesis of FIG. 1A. FIG. 1C is an enlarged cutoff explosive view of a small segment of the laminated composite hip prosthesis of FIG. 1A showing the orientation of the plies of the composite. According to FIG. 1C, the prosthesis is a balanced composite hip prosthesis. The prosthetic device has an elongated direction along the shaft of the prosthesis. The orientation of the fiber composites is denoted as positive angle along a counterclockwise direction and a negative angle along a clockwise direction. The neck is therefore oriented along an acute positive direction. The balanced composite is fabricated by laminating a plurality of layers as that shown in FIG. 2. The composites are fabricated by laminating multiple layers wherein each layer is generally referred to as a ply and the fibers in each layer is called the ply orientation of the layer. The sequence of ply orientation of the plies in a composite through the thickness is referred to as stacking sequence. Laminated composite structures can be categorized according to their stacking sequence. The laminates, which are constructed by placing the laminae symmetrically with respect to the mid-plane, are often termed symmetric laminates. The ply orientations of the layers on one side of the mid-plane are a mirror image of the ply orientations on the other side. The symmetric laminates are commonly constructed to simplify their analysis and to eliminate the bending inplane coupling. When all of the plies in a laminate having a counter ply with an opposite sign, as that shown in FIGS. 1 and 2, the laminate is referred to as balanced. A balanced laminate has equal number of plies with positive and negative orientations while the unbalanced laminate does not have equal numbers of plies that have positive and negative orientations to offset each other.
In U.S. Pat. No. 5,064,439, (issued on Nov. 12, 1991) entitled "Orthopedic Device of Biocompatible Polymer with Oriented Fiber Reinforcement", Chang et al. disclose an orthopedic device such as a hip stem with longitudinal curved body fabricated with biocompatible polymer with oriented fiber reinforcement. The reinforcing fibers are continuous filament fiber plies with parallel oriented fibers in each ply. The plies are curved longitudinally to approximately correspond to the curve of the body. The fiber orientation is balanced by providing a ply of negatively angled offset fibers of similar angle for each positively angled offset ply. The device is made by molding plies preimpregnated with polymer prepregs simultaneously by molding a plurality of prepregs into segments which are then molded together, or by molding a segment and incrementally molding additional layers of prepregs thereto in a series of progressive lager molds. This "balanced" device has the difficulties that stress and deformations caused by unbalanced or side-dependent loading are not taken into account for designing and applying the prosthetic devices.
Salzstein et al. disclose in another U.S. Pat. No. 5,163,962 entitled "Composite Femoral Implant Having Increased Neck Strength" a femoral implant for a hip prosthesis using a longitudinal shaft having a neck extending therefrom at an acute angle .theta. to the longitudinal direction. The prosthesis is made of layers of carbon fiber in a polymeric matrix, each layer containing unidirectional fiber and the layer arranged such that carbon fibers are oriented in the longitudinal direction and the .+-..theta. direction. The improvement of this invention involves balancing at least 50% of the layers in the .+-..theta. direction. An example of the stacking sequence is defined as [-18.sub.i,+18.sub.i, +40.sub.i,-40.sub.i, 0.sub.i, +40.sub.i, -40.sub.i, 0.sub.i ].sub.ns where ns is used to indicate that it is not symmetrical to the mid-plane layer. Such implant is still a balanced composite and does not offer a solution to the side dependent effects discovered in the hip replacement prostheses.
Dumbleton et al. disclose in another U.S. Pat. No. 5,181,930 (issued on Jan. 26, 1993), entitled "Composite Orthopedic Implant", a beam adapted for implantation within a bone for supporting bending and torsional loading forces. The beam has a stiffness defined by a modulus elasticity wherein the stiffness varies along the length of the beam to match the corresponding stiffness of the cortical bone adjacent the beam after implantation within the bone. The beam is made with elongated core formed of continuous filament carbon fibers embedded in a thermoplastic polymer matrix with the carbon filaments extending in a direction substantially parallel to the longitudinal axis of the beam.
Balanced fiber composites are employed in these patents. As explained earlier, when the implanted device is for a side-dependent operation such as a hip implant, the prosthetic device are subject to anisotropic forces which depends on the right or left sides of the implant The side dependent effect can cause mismatches and generate micromotions and stress shielding if the prosthetic device is designed without taking the side-dependent effects into considerations. The orthopedic implant devices mentioned above which uses balanced fiber reinforcement for fabrication cannot resolve these difficulties.
In another U.S. Pat. No. 5,522,907, Moran et al. disclose an unbalanced composite where two fiber orientations are used to form an unbalanced composite femoal implant for a hip prosthesis. Moran's prosthesis is made with at least 50% of the fibers of the composite pliers oriented in the .theta. direction that is the direction of the neck relative the longitudinal direction aligned with the shaft as that described in column 2, lines 24-33. In one example, as that presented in column 6, lines 7-30, Moran et al. show that when the neck is 40.degree. relative to the shaft, the fibers are arranged to have at least 50% to be oriented at 40.degree.. The angle of the neck is independent of side. According all the independent claims of Moran, the fibers are oriented to align with the direction of the neck. By arranging at least 50% of the fibers to orient along the neck direct .theta., the prosthetic device of Moran does not address issues caused by the side-specific load and stress conditions. Specifically, Moran's device does not address the difficulties caused by a twisting and rotating axial-loading that is side-dependent. Due to the fact that the relative angle .theta. of the neck angle relative to the shaft is substantially the same for left and right side application, there is very little correlation with the side-specific loading conditions. Therefore, the device disclosed by Moran cannot resolve the problems of stress and deformations caused by relative micro-motions resulted from the side-dependent twisting and rotational axial loading.
Therefore, there is still a need in the art of prosthetic device design and manufacture by the use of composites to provide improved techniques to overcome these difficulties. Specifically, the new design and manufacture techniques must provide a prosthetic device which can offset the side dependent unbalanced effects to minimized the mismatches encountered in the conventional orthopedic implants for applications such as total hip replacement.